Stillwellite-(Ce) (Ce; La; Ca)BSiO5 c 2001 Mineral Data Publishing, version 1.2 ° Crystal Data: Hexagonal. Point Group: 3: As °at rhombohedral crystals, to 4 cm, and massive. Twinning: Observed about [100]. Physical Properties: Cleavage: One imperfect. Fracture: Conchoidal. Hardness = 6.5 D(meas.) = 4.57{4.60 D(calc.) = 4.67 » Optical Properties: Transparent to translucent. Color: Red-brown to pale pink; colorless in thin section. Streak: White. Optical Class: Uniaxial (+) to biaxial (+). ! = 1.765{1.784 ² = 1.780{1.787 2V(meas.) = 0±{6± Cell Data: Space Group: P 31: a = 6.841{6.844 c = 6.700{6.702 Z = 3 X-ray Powder Pattern: Mary Kathleen mine, Australia. 3.43 (s), 2.96 (s), 2.13 (ms), 4.44 (m), 1.864 (m), 2.71 (mw), 2.24 (mw) Chemistry: (1) (2) (1) (2) (1) (2) SiO2 22.40 22.06 La2O3 27.95 19.12 MgO 0.06 UO2 0.22 Ce2O3 33.15 30.82 CaO 0.95 0.34 ThO2 5.41 Pr2O3 1.82 F 0.30 + B2O3 12.23 [13.46] Nd2O3 5.36 H2O 0.85 Al2O3 0.42 Sm2O3 0.34 H2O¡ 0.10 Y2O3 0.74 0.28 Fe2O3 0.18 P2O5 0.67 Total [100.00] [99.26] (1) Mary Kathleen mine, Australia; recalculated to 100.00% after removal of very small amounts of uraninite and apatite determined by separate analysis. (2) Vico volcano, near Vetralla, Italy; by electron microprobe, B2O3 calculated from stoichiometry, original total given as 99.23%; corresponds to (Ce0:50La0:31Nd0:08Th0:05Pr0:03Ca0:02Sm0:01)§=1:00B1:02Si0:97O5: Occurrence: Locally abundant as a metasomatic replacement of metamorphosed calcareous sediments (Mary Kathleen mine, Australia); in alkalic pegmatites in syenite in an alkalic massif (Dara-i-Pioz massif, Tajikistan). Association: Allanite, garnet, uraninite (Mary Kathleen mine, Australia); calcite, monazite, bastnÄasite, thorite, uranothorite, thorianite (Desmont mine, Canada); pyrochlore, tienshanite, sogdianite, thorite, cesium kupletskite, reedmergnerite, steacyite, pectolite, quartz (Dara-i-Pioz massif, Tajikistan). Distribution: In the Mary Kathleen mine, about 55 km east of Mt. Isa, Queensland, Australia. At the Desmont mine, near Wilberforce, Ontario, and at Mont Saint-Hilaire, Quebec, Canada. From Mineville, Essex Co., New York, USA. Large crystals in the Dara-i-Pioz massif, Alai Range, Tien Shan, Tajikistan. From the Inagli massif, 30 km west of Aldan, Yakutia, Russia. Found around the Langesundsfjord, Norway. In the Il¶³maussaq intrusion, southern Greenland. At Bassano Romano and on the Vico volcano, near Vetralla, Lazio, Italy. Name: For Dr. Frank Leslie Stillwell (1888{1963), Australian ore mineralogist, and its predominant cerium content. Type Material: National Museum of Natural History, Washington, D.C., USA, 113038. References: (1) McAndrew, J. and T.R. Scott (1955) Stillwellite, a new rare-earth mineral from Queensland. Nature, 176, 509{510. (2) (1956) Amer. Mineral., 41, 370 (abs. ref. 1). (3) Gay, P. (1957) An X-ray investigation of some rare-earth silicates: cerite, lessingite, beckelite, britholite, and stillwellite. Mineral. Mag., 31, 455{468. (4) Dusmatov, V.D., A.F. E¯mov, and Y.I. Semenov (1963) First discoveries of stillwellite in the USSR. Doklady Acad. Nauk SSSR, 153, 913{915 (in Russian). (5) Dusmatov, V.D. (1964) First discovery of stillwellite in USSR. Dokl. Akad. Nauk Tadzh. SSR, 7, 33{35. (6) Callegari, A., G. Giuseppetti, F. Mazzi, and C. Tadini (1992) The re¯nement of the crystal structure of stillwellite: RE[BSiO5]: Neues Jahrb. Mineral., Monatsh., 49{57. (7) Burns, P.C., F.C. Hawthorne, D.J. MacDonald, G.D. Ventura, and G.C. Parodi (1993) The crystal structure of stillwellite. Can. Mineral., 31, 147{152. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise without the prior written permission of Mineral Data Publishing..